Preparation of thiolesters



Patented July 13, 1948 PREPARATION or 'rmonas'rnas Chester M. Himel,Bartlesville, Okla., aslignor to Phillips Petroleum Compan'macorporation of Delaware No Drawing.

Application February 12, 1946, Serial No. 647,191

6 Claims. (Cl. 260-455) 1 This invention relates to a process fortheprep= aration of thiolesters and more particularly thiolesters oftertiary aliphatic mercaptans having 8 or more carbon atoms permolecule. More speciilcally the invention relates to a method for thesynthesis of thiolesters by the interaction of anhydrides of organicmonobasic acids with tertiary aliphatic Ca and higher mercaptans.

The principal object of my invention is to provide a novel and usefulprocess for the preparation of thiolesters of tertiary aliphatic Cs andhigher mercaptans. Another object is to prepare thiolesters which haveimproved stability toward saponiflcation reactions and which are usefulas plasticizingand modifying agents for synthetic and natual rubbers andresins. An-

other object is to provide a process for the preptiary aliphatic Ca andaration of thiolester from tertiary aliphatic Ca and heavier mercaptansand the anhydrides of organic monobasic acids in the presence of acidictype catalysts. Another object is to provide a process which utilizesreadily available tertiary aliphatic Ca and higher mercaptans and whichalso utilizes anhydrides of organic monobasic acids to preparethiolesters possessing extremely high utility as plasticizers and aschemical intermediates. Many other objects will more fully appear from aconsideration of this description. I have found that thiolesters oftertiary aliphatic Ca and higher mercaptans possess many advantages overthe other types of esters when they are employed as plasticizing agentsfor natural and artificial rubbers, various alkyd resins and numerousother polymeric materials. For example, thiolesters prepared fromtertiary aliphatic Ca and higher mercaptans show a remarkable resistancetoward hydrolytic reactions while other esters are known to undergohydrolysis with relative ease. The superiority of the thiolesters madein accordance with the present invention is also manifested in theirincreased compatibility toward many natural and synthetic resinousmaterials. While the reasons for these improved properties are notclearly understood,

it is probable that the presence of the sulfur atom in the esterstructure and also the highly branched nature of the carbon chain of thetertiary aliphatic Ca and higher mercaptans utilized are at leastpartially responsible for the unexpected properties of the compoundsherein dc where R may be an alkyl group from methyl to those with longhydrocarbon chains, as well as aralkyl and aryl radicals, and R is atertiary alkyl group containing at least 8 carbon atoms.

According to the process of this invention, terhigher mercaptans may bereadily converted to thiolesters by the interaction of said mercaptanswith organic acid anhydrides of monobasic acids in the presence ofacidto type catalysts such as sulfuric acid, phosphoric acid,hydrochloric acid, zinc chloride. aluminum chloride, boron fluoride andthe like. Although it is well known that tertiary mercaptans show apronounced tendency to undergo degradative decomposition reactions withcleavage of hydrogen sulfide under the influence of heat or the actionof acids, I have found that reaction conditions may be controlled insuch a way that thiolester formation is the predominant reaction. Thus,it has been found advisable to "carry out the reaction ciently low toprevent substantially all mercaptan decomposition and at the same timemain-' dride and the niercaptan chosen.

I have found that, under the specific conditions chosen and in thepresence of acidic type catalysts, a smooth and rapid reaction betweentertiary aliphatic Ca and higher mercaptans and organic monobasic acidanhydrides will occur with the resultant formation of thiolesters.Processes have been described for the preparation of thiolesters frommercaptans, particularly the primary mercaptans of relatively lowmolecular weight, in which it is essential that one of the reactionproducts be removed as soon as it is formed. Such a separation step isaccomplished by azeotropic distillation, fractional distillation,

extraction or other means. According to my.

process satisfactory yields of the'desired esters at temperatures summaybe obtained in a batchwise operation without the necessity of removingone of the reaction products. Thus the disadvantage of intro- '4 ingdata are representative of the physical properties of some of thetertiary mercaptans from such sources.

Pnoperties of tertiary mercaptans ducing an additional step in theprocess is eliminated and thereby a marked reduction in op- .eratingcosts is effected. Inv my process the reaction mixture is agitated at acontrolled temperature and for a suflicient length of time to in- ,surean optimum yield oi the thiolester after which the material is waterwashed, dried and distilled. The process, therefore, provides aconvenient and economical method for the synthesis 01' an importantgroup of sulfur compounds.

The process in one of its more specific embodiments may comprise theslow addition of a tertiary aliphatic Cs or higher mercaptan to asolution containing an acidic type catalyst and an organic acidanhydride of a monobasic acid. The mol ratio of anhydride to mercaptanis generally at least oneto one with an excess of the anhydridepreferred in order to effect substantially complete conversion of themercaptan. While it is preferable to use an excess of the anhydride,certain limitations are imposed on account of the cost of said materialwhen large scale operation of the process is practiced. Therefore, theratio of reactants employed in a given synthesis will be governed by theavailability and cost of the materials. Any conventional type reactorprovided with a means for stirring or therwise agitating the reactionmixture is suitable for practicing the present process. Temperatures inthe range of 0 to 200 C. are applicable with the somewhat narrower rangeof 40 to 90 C. being preferred. The temperature chosen is governedlargely by the stability and reactivity of the mercaptan beinesterified. In general the higher the molecular weight of the mercaptanthe lower the reaction temperature utilized.' In the interest ofoperating efficiency, it is important to adjust the reaction conditionsso that the ester is formed before appreciable decomposition of themercaptan occurs. Subsequent to the addition of the tertiary Ca orhigher aliphatic mercaptan, the reaction mixture is agitated while thetempertaure is held within the optimum range for a period of from aboutone to about three hours or until the reaction is complete. The productis then water washed, dried and distilled.

The mercaptan intermediates of the present process comprise the tertiarytypes having from 8 to 20 or more carbon atoms per molecule. While inspecialized cases pure individual mercaptans may be desirable, for manypurposes commercially available mixtures of isomers are satisfactory.

2,392,555. In most cases, the tertiary mercaptans are mixtures ofisomeric compounds. The follow- Other high molecular weight tertiaryaliphatic mercaptans which may be utilized in practicing the presentinvention are: tertiary C10 mercaptan, tertiary hexadeeyl mercaptan,tertiary octadecyl mercaptan and tertiary eicosyi mercaptan. Tertiaryaikyl mercaptans having more than 20 carbon atoms per molecule maylikewise be employed.

Anhydrides which are applicable in the thiolester synthesis hereindescribed range from the low molecular weight anhydrides of aliphaticmonobasic acids; such as acetic anhydride, to compounds containing longhydrocarbon chains or branched chains. Anhydrides of aromatic acids,such as benzoic acid and its derivatives, as well as anhydridescontaining aralkyl groups are also within the scope of this disclosure.

Catalysts which have been shown to be applicable to the reaction includesulfuric acid, phosphoric acid, hydrogen chloride, hydrogen fluoride.zinc chloride, aluminum chloride, stannic chloride, zinc sulfate, sodiumbisulfate, boron fluoride, boron fluoride-phosphoric acid complex, boronfluoride etherate, benzene sulfonic acid and other aromatic sulfonicacids. In the presence of these catalysts, thiolesters can be preparedby reaction of tertiary mercaptans with either aliphatic or aromaticacid anhydrides.

Following are non-limiting specific. examples of the practice of myinvention.

Example I Tertiary dodecyl thiolacetate was prepared in the followingmanner: Acetic anhydride (60 parts by weight) and freshly fused zincchloride (10 parts by weight) were charged in a flask equipped with astirrer and a thermometer. Tertiary C12 mercaptan (101 parts by weight)was added dropwise maintaining the temperature in the range of 50-60 C.The reaction was held with agitation at 70 C. for one hour after theaddition of all of the mercaptan. The product was washed with water,dried, and distilled to give 92 parts by weight of tertiary dodecylthiolaoetate b. 95-105 C./2 mm. The refractive index or the ester was 111.4780. 1

Example II with aqueous sodium carbonate. with water, was

dried and then distilled. The thiolester product parts by Weight)distilled at 130-145" C./2 mm.

From the foregoing description it will be seen that my inventionprovides a simple and economical method of preparing very valuablethiolesters of higher-boiling tertiary aliphatic mercaptans. Manyadvantages of the process of my invention will be immediately apparentto those skilled in the art. An important advantage is that cheap andreadily available raw materials are utilized to give products or highutility in the arts. Another advantage is that no means need be providedfor removing a reaction product or products as soon as formed. Anotheradvantage is that the valuable thiolester product is formed beforeappreciable decomposition of the tertiary merca n occurs, whereby a goodyield based on the rawmaterial taken is obtained. Numerous otheradvantages wiiLbe appreciated by those skilled in the art.

I claim:

1. The process of preparing a thiolester having the general formulaa-c-s-w where R is a radical selected irom the group consisting of alkyland aryl radicals having not more than 6 carbon atoms and where R. is atertiary alkyl radical having at least 8 carbon atoms, which comprisesslowly adding a tertiary alkyl mercaptan having at least molecule .toa'solution containing an organic acid anhydride a monobasic organiccarboxyllc acid having not more than 7 carbon atoms per molecule. and=anacidic catalyst. agitating said solution during said addition andmaintaining the reaction mixture during said addition at a temperaturewithin the range oi from 40 to 90 C., the moi ratio oi said anhydride tosaid mercaptan being slightly greater than one to one, maintaining theresulting mixture at a temperature withinthe mascot from 40 to 90 C. fora period of time of 1mm one tothree hours while agitating same, andrecovering said thiolester from the resuiting mixture.

2. The process of captan has not more jmoiecule.

claim 1 wherein said merthan 20 carbon atoms per 8 carbon atoms per 3.The process of claim 1 wherein said oraan acetate which comprises slowlyadding tertiary dodecyl mercaptan to a mixture of a major proportion ofacetic anhydride and a minor propor- 60 Number tion of fused zincchloride while agitating said mixture and maintaining it at atemperature in the range of to said mercaptan only until an amountslightly less than equimolar to said acetic anhydr-ide has beenintroduced, holding the resulting mixture with agitation at C. for onehour after the addition of said mercaptan has been discontinued. andrecovering tertiary dodecyl thioiacetate from the resulting reactionmixture.

6. The process of making tertiary tetradecyl thioibutyrate whichcomprises slowly adding tertiary tetradecyl mercaptan to a mixture of amajor proportion of butyric anhydride and a minor proportion of benzenesuiionic acid while agitating sa-id mixture and maintaining it at atemperature of 50 C., continuing the addition of said mercaptan onlyuntil an amount slightly less than equimolar to said butyric anhydridehas been introduced, holding the resulting mixture with agitation at 50C. 1dr three hours after the addition of said mercaptan has beendiscontinued,.

and recovering tertiary tetradecyl thiolbutyrate from the resultingreaction mixture.

CHESTER M. HIMEL.

REFERENCES crran The following reierences are of record in the tile ofthis patent:

UNITED STATES PATENTS Name Date Allen Aug. 27, 1940 Kendall Nov. 20,1945 OTHER REFERENCES Chakravarti, Chemical Abstracts. vol. 21, page C.,continuing addition of

